The processing of hazardous or offensive waste materials produced by municipalities and industries has reached critical importance in modern-day society. Concern for the quality of life and the environment have compelled governmental agencies to promulgate legislation to ensure that future generations will not suffer from the excessive wastes of our present day society. Under the regulations which implement these governmental edicts, waste must be discarded in a fashion which is non-offensive and presents little or no threat to the air, water and land upon which the waste is ultimately disposed. The United State Congress in 1976 enacted Subtitle C of the Resource Conservation and Recovery Act (RCRA), Pulic Law 94-580, for the purpose of instituting a national hazardous waste control program similar in function to the previously promulgated Air Pollution and Water Pollution Control programs.
The United States Environmental Protection Agency, charged with the responsibility for implementing and supervising the hazardous waste control program called for under RCRA, promulgated in 1980 a series of regulations which require that certain sludges, slurries and other liquid wastes containing specified hazardous materials may no longer be deposited in landfills without pre-treatment, stabilization, and dewatering. Wastes must additionally have acceptable toxicity levels as measured by certain established tests before they can be discarded in a landfill. One primary objective of these governmental requirements is to achieve a non-flowing consistency of the waste by reduction of the liquid content or increase of the solid content to eliminate the presence of free liquids prior to final disposal in the landfill. The end result of this and other similar legislation and regulations is that many liquid or semi-liquid wastes containing hazardous materials will require dewatering, chemical fixation, solidification, or some combination thereof, prior to ultimate disposal.
Chemical fixation and solidification processes have found recent favor for detoxifying hazardous materials and for producing solid wastes having physical properties suitable for ultimate disposal in landfills, ocean dumping, etc. For example, U.S. Pat. No. 3,837,872 discloses a method for treating liquid wastes by adding an aqueous solution of an alkali metal silicate and a silicate setting agent, which converts the waste into a chemically and physically stable solid product. The patent to Thompson, U.S. Pat. No. 3,980,558, discloses a method for treating liquid wastes by adding a solidification agent consisting essentially of hydraulic cement.
The terminology of chemical fixation and solidification has not been consistent in the prior art due primarily to the fact that until recently most of the waste treatment systems offered were considered proprietary. Such terms as "encapsulation", "crystal capture", and "pseudo mineral" often appear in the prior art instead of discussions concerning the operations of such systems, most likely because the actual chemical reactions involved are complex and not completely understood.
There is also a tendency in the prior art to confuse the terms "chemical fixation", "stabilization", and "solidification". "Stabilization" is essentially a pretreatment process which alters wastes to prevent further chemical reactions, e.g., the use of lime in biological sludges to kill or inactivate micro-organisms so that the sludge will not undergo further biological decomposition. "Chemical fixation" refers to the chemical technology used to destroy, de-toxify, immobilize, insolubilize, or otherwise render a waste component less hazardous or less capable of finding its way into the environment. The term often denotes a chemical reaction between one or more waste components in a solid matrix, either introduced deliberately or preexisting in the waste. For example, the ion exchange of heavy metals within the alumino-silicate matrix of a cementitious solidification agent is a chemical fixation. There is a wide variety of chemical fixation techniques known in the art for preparing waste residues for solidification, encapsulation or disposal without solidification.
The term "solidification" is the transformation of a waste residue into a solid physical form which is more suitable for storage, burial, transportation, ocean disposal, or re-use in processes such as highway paving or topping for a landfill. Solidification may reduce the hazard potential by means of creating a barrier between the waste particles and the environment, limiting permeability of the waste to water, or reducing the affected surface area of the waste available for diffusion. There are various types of solidification known in the art which do not incorporate chemical fixation. Moreover, the solidification of waste does not always involve a chemical process, e.g., drying, dewatering and filtration are physical processes which are sometimes considered "solidification".
Conventional chemical fixation and solidification techniques sometimes do not adequately treat wastes. Generally, these prior art fixation and solidification techniques are unsuitable for sludges and slurries containing a low percentage of solids, for example, less that ten to twenty percent by weight. Dewatering processes frequently cannot achieve a true solid and are sometimes subject to reversion to the original state by the simple addition of water. Pure absorption processes such as the addition of clays or lime suffer also from the problem of reversion to the original state. Moreover, in some wastes, the absorbed liquid phase of the waste can be squeezed out of the "solidified" material under mechanical pressure such as may occur in a landfill or even during the handling or transporting process.
The nuclear industry in the 1950's recognized the need for preventing the reversion of wastes into a liquid phase. Early methods in this industry employed simple absorption techniques such as the addition of vermiculite, or solidification by making a concrete mixture with very large quantities of Portland cement. Large quantities were required to assure that there would be no free standing water after curing of the cement. This inevitably resulted in a relatively large ratio of cement to waste, and a large volume of end waste product which must be transported and disposed. Substantial volume increases can make disposal prohibitively expensive in landfills which calculate disposal prices by volume.
Moreover, the Nuclear Regulatory Commission has stated in a preliminary draft of 10 C.F.R. Part 61 that any nuclear wastes containing liquids must be immobilized by solidification to an end product in a dry, free-standing, homogeneous, monolithic matrix which is not readily dispersible, friable, or soluble, and which contains not more than 0.5% or one gallon per container, whichever is less, of noncorrosive liquids. Under these standards, liquids that have been immobilized by only the addition of absorbent materials such as diatomaceous earth or vermiculite are not acceptable waste forms.
The cement-silicate solidification process such as disclosed in U.S. Pat. No. 3,837,872 referenced above is designed to provide a solidification waste treatment method which does not allow reversion to the liquid phase and which possesses a reduced volume of end product. The method is usable with a wide variety of wastes including those emanating from manufacturing, metal producing operations, and the like, which contain large concentrations of toxic, polyvalent metals. This cement-silicate technology was developed primarily for use with water-based, primarily inorganic wastes with low to moderate solids content (1-30%). The technology was specifically designed for use with continuous processing equipment wherein a liquid silicate solution can be added in a controlled manner so as to control the set or "gel" time. The gel time is controlled by the concentrations of cement and liquid silicate as well as the composition of the waste. In many applications, liquid silicate solidification systems have such short gel times that setting begins before the mixed waste leaves the processing equipment.
A problem with conventional cement-liquid silicate solution solidification treatment processes is that the two components of the system must be added to the waste separately since pre-mixing of such waste treatment materials would result in immediate setting thereof. The rapid setting rate of a cement and liquid silicate solidification system, together with the fact that the components must be added separately, makes the system usable only with continuous processes and very difficult to use in batch waste treatment.
The use in the prior art of a dry soluble silicate instead of a liquid silicate solution together with cement for waste treatment creates a different problem. This type of waste treatment requires more time for the gel reaction to occur since the silicate must be solubilized before it can gel. During this time, some settling of the sludge may occur in batch-processing treatment facilities or in continuous processes with low flow rates or inadequate agitation. if there is settling of the sludge prior to gel, free-standing water will occur on the top of the waste, which renders the treatment incomplete and unsatisfactory.
Another problem with the conventional cement-silicate method of waste solidification is that it is sensitive to certain waste constituents which act as inhibitors or otherwise interfere with the solidification process. The interactions between the waste constituent and the chemicals are extremely complex because many different reactions occur simultaneously, especially with wastes containing a variety of reactive pollutants.
Three general classes of interactions which have been identified include (1) reactions between the sodium silicate and the waste being treated, (2) reactions between the silicate and certain reactive components such as the calcium ion of the Portland cement, and (3) the hydrolysis and hydration reactions of Portland cement itself. These reactions and the ability of the resultant end product to encapsulate and hold waste constituents are discussed more fully in U.S. Pat. No. 3,837,872, the disclosure of which is incorporated herein by reference.
Chemical waste constituents which have been identified as solidification inhibitors fall within two basic categories: inhibitors of the cement-setting reaction, and inhibitors or precipitators of the silicate or of the cement-silicate mix. Some known cement setting inhibitors include borates; phosphates; sulfide ions; sodium arsenate; sulfates in high concentrations; oil in high concentrations; certain metal salts including lead, zinc, and copper; organics in various concentrations; and very finely divided particulate matter. Inhibitors or precipitators of the silicate or of the cement-silicate mix include ammonia or ammonium compounds, active anaerobic conditions, high concentrations of aromatic organics, pH conditions less than 4, nitrates, high concentrations of metal ions, and water soluble organics.
When any of the known inhibitors or precipitators are present in the waste being treated, the gel time of the cement-silicate mix is more difficult to predict and control. Often, free-standing water will be found on the top of the solidified waste when precipitation has occurred or when settling of certain waste constituents in the sludge occurs prior to gelling or setting. This free-standing water is a significant problem in conventional alkali metal silicate-cement solidification systems since the water can contain toxic substances in solution.